The present invention relates to fiber optic continuity test systems and, more particularly, to a single-ended tester capable of detecting discontinuities in an optical fiber.
Laser initiated ordnance (LIO) systems are well known in the art and typically employ a light pulse which is passed along a fiber optic cable and caused to impinge on an energetic material to heat it to ignition. Laser initiated systems are safer than electrical initiation systems in that the former are not susceptible to inadvertent initiation by static or stray electromagnetic radiation. In addition to avoiding accidental operation, however, ordnance systems are also required to reliably operate upon occurrence of a predetermined stimulus. Continuity of a firing channel must therefore be confirmed to determine whether the channel is misaligned, contaminated, mismated, severed, crushed or otherwise damaged. Without a test capability, the only available information relating to that damage is that, upon sending a "firing" light pulse, the ordnance does not initiate. Fiber optic continuity test systems are usually either single ended or dual ended, with singled ended systems being employed in LIO systems because access to only one end of the fiber is possible.
Many single-ended-testers utilize optical time domain reflectometry (OTDR). OTDR systems work by first transmitting pulses of light into a fiber and then measuring the light that is reflected back using sophisticated high speed detection and timing electronics. The time that it takes for the reflected light to return corresponds to the distance it travels along the fiber. This allows the OTDR system to produce a fiber signature. Two types of reflections occur. Pulse reflections are generated at breaks or joints where the light pulse encounters something other than a continuous glass core. In a typical LIO system, pulse reflections would occur where two sections of fiber-optic cable are connected, and at the interface between the end of the fiber-optic cable and the ordnance. Back scatter reflections are generated uniformly along a fiber as the transmitted pulse travels through the fiber. The back scatter signal provides a measurement of fiber attenuation. OTDR systems are frequently used for finding breaks in communication cables which are typically several kilometers long. One-half meter is considered excellent resolution for an OTDR system. In LIO systems, however, one meter resolution is not acceptable because a break close to the fiber/ordnance interface could not be distinguished from the end of the optical fiber by an OTDR system (e.g., a break only a millimeter from the fiber/ordnance interface would disable the laser ordnance system but would not be detected by an OTDR system). This difficulty is magnified by the from that the fiber/ordnance interface is a high stress region and is an area where cracks are likely to form.
Where the resolution of OTDR systems is unacceptable, fiber optic continuity systems employing a dichroic mirror have been utilized. U.S. Pat. No. 5,270,537 teaches a continuity test system employing a dichroic filter (at the fiber/ordnance junction) which reflects light within one wavelength range for continuity test proposes and transmits light within a second wavelength range for ignition purposes. There is a fiber optic conduit having a plurality of connectors contained therein, where the fiber-conduit connects the light sources with the ordnance device. The system tests the integrity of the optical fiber by shining a test laser into the fiber-optic cable, a portion of the light reflects as it passes each of the plurality of connectors. Each of these reflections travels to a detector through the fiber-optic cable and is detected. The majority of the test laser light which remains unreflected continues down the fiber-optic cable is reflected by the dichroic coating. The reflection of the test laser is also sent back up the fiber-optic cable and is detected. The system must be calibrated to distinguish between the reflections which occur at each connector, and the dichroic reflection, i.e., the system must determine the amount of light that must be reflected by the dichroic mirror to ensure there are no breaks in the fiber optic cable. In theory, if there is a break in the fiber-optic cable, the amount of light which transmits through the break, and is subsequently reflected by the dichroic mirror will be at a low level. The detector will detect this low level and determine that there is a break in the fiber-optic cable.
U.S. Pat. No. 5,359, 192, entitled "Dual-wavelength Low-power Built-in-test For a Laser-initiated Ordnance System" teaches another continuity test system employing a dichroic filter having a wavelength-dependent reflectivity. A fiber-optic-conduit having a plurality of connectors (three or more) connects the light sources with the ordnance device, and the dichroic filter is placed at the interface of an ordnance device and the optical fiber. The system tests the integrity of the optical fiber by shining two different wavelengths of test-light into the fiber and detecting the light reflected by the dichroic mirror. A relative comparison is made of the light reflected by the wavelength-dependent dichroic mirror of the two different wavelengths of light, and optical continuity is confirmed if more light will be reflected by the mirror at one of the wavelengths than the other. This scheme was developed to overcome the prior art deficiencies of trying to differentiate the reflections between the conduit-connectors and the dichroic mirror reflections because the connector-reflections will have a substantially fiat optical response within a band encompassing the two wavelengths and therefore do not contribute to the differences in the intensities of the reflected light.
Thus, it is desirable to provide a simple and reliable single-ended apparatus for ascertaining fiber optic link continuity.